1. Field of the Invention
The present invention relates to a binder for powder metallurgy capable of being suitably incorporated in a raw powder for powder metallurgy, such as iron powder or copper powder, also relates to a mixed powder for powder metallurgy, and further relates to a method for producing the mixed powder for powder metallurgy.
2. Description of the Prior Art
A mixed powder for powder metallurgy is generally produced by mixing an iron powder-based metal powder with an alloying powder such as graphite, nickel, copper, or molybdenum, and then treating the resulting powder mixture with a binder. The binder treatment permits prevention of segregation of components caused by the difference in specific gravity between the metal powder and the alloying powder such as graphite, and suppression of scattering of graphite, etc.
As one example, Japanese Patent No. 1597077 discloses a binder treatment method comprising the steps of mixing a solution type binder with a raw powder, which binder comprises a vinyl acetate homopolymer or a polyester resin dissolved in a solvent, and then removing the solvent by volatilization. As another example, JP-B No. 89364/1994 discloses a binder treatment method comprising melt-mixing zinc stearate or a wax with a raw powder for subsequent cooling.
Treating a raw powder for powder metallurgy with a binder and allowing the binder to fulfill its function to a satisfactory extent needs to first mix binder components and a raw powder intimately. From this standpoint, a solution type binder of a low viscosity is used as the binder. In this case, for minimizing the influence of the solvent used in the solution type binder on physical properties of the final powder metallurgy product, the solvent is removed by volatilization from the mixed powder. From the standpoint of easiness of solvent removal by volatilization and solubility of the base resin in the binder, such a solvent as toluene or acetone is used as the solvent for the binder. However, these solvents are highly flammable and such a danger as fire is involved in the manufacturing process for a mixed powder for powder metallurgy. Moreover, in view of the recent demand for coping with environmental problems, it is required to decrease the amount of the solvent used.
Poor performance of the binder for powder metallurgy used might not only bring about the foregoing segregation of components or scattering of graphite results, but also badly influence characteristics of the resulting mixed powder. Further, for example, the binder needs to possess heat resistance at a warm molding temperature in the range from room temperature to 200° C. or so, and at the same time needs, in sintering, to decompose thermally with ease and afford a good molded product of powder metallurgy without any residue.